The goal of this project is to understand protein evolution at the molecular level. An interdisciplinary approach will be taken that combines the strengths of evolutionary biology and molecular biophysics. As proteins evolve, they must maintain function or they will be removed by selection. Recent investigations using ancestral protein resurrection have found functional intermediates in the evolution of the medically important steroid receptor (SR) proteins. The mutational pathway connecting different functional states is characterized by strong sign epistasis: "permissive" mutations must precede "function-switching" mutations to maintain function along the evolutionary trajectory. The mechanism of these permissive mutations is not well understood. This project will use the methods of biophysics and directed evolution to determine the mechanisms of these historical permissive mutations. The first aim of the project is to probe the relationship between protein stability and evolution. The second aim is to understand the role of contingency in the evolution of complex protein function. This work has important implications for understanding the general principles of protein evolution. It will also provide insight into the molecular determinants of ligand specificity in SRs, which will inform attempts to create novel therapeutic agents that modulate SR function. PUBLIC HEALTH RELEVANCE: Steroid receptors-which are key players in diseases ranging from asthma to cancer-must discriminate between similar hormones to function properly. Like all proteins, their ability to perform this function is a product of their evolutionary history. This project attempts to understand how this function evolved, which will potentially allow for new drugs to modulate steroid receptor function to treat disease.